Introduction

Abstract

One measure of progress in research on interstellar matter is our expanded awareness of the broad variations in the physical state of material dispersed throughout our galaxy. As they become recognized, the various regimes of density, temperature and ionization are often characterized as discrete “phases,” although in some circumstances the boundaries between such phases may not be as well defined as we once thought. At one end of the spectrum of conditions are the compact gas clouds, rich in dust grains and complex molecules, having temperatures below a few tens of degrees K and densities in excess of 10−19 g cm−3. If we exclude suprathermal particles (cosmic rays), we may identify the other extreme as collisionally ionized gas with a temperature on the order of 105 to 107 K and a mean density of around 10−27 g cm−3.